In traditional stationary wireless communication systems, subscriber units synchronize to base stations and continuously track them. In time-domain multiplex systems, transmit and receive timeslots dictate precise time and frequency alignment between a transmitting subscriber unit and a receiving base station. Due to inherent propagation delays between a subscriber unit and a base station, a subscriber unit must develop a transmit time advance offset to compensate for the propagation delay between a subscriber unit and a base station. A precise offset value allows the transmitted information to arrive at the base station during the proper framing or receive window time.
In communication systems where base stations are in substantial motion, such as airborne base stations (e.g., orbiting satellites), the propagation delay between a subscriber unit and an orbiting satellite does not remain constant due to varying distances between a satellite and a subscriber unit. In systems where the base stations are very dynamic, such as with a constellation of low earth orbiting satellites, satellites within communication range of a subscriber unit exhibit a continuously varying propagation delay over a relatively large range of values.
To accommodate a transmission between a subscriber unit and a satellite, a subscriber unit must approximate a transmit time advance to employ when attempting to deliver information to a satellite during a receive window. When nodes in a communication system are static, a subscriber unit may simply maintain a static time advance offset value for repeated use. In terrestrial systems, the range of propagation delay values is necessarily limited by the radio horizon and nominal advance values are adequate, even though some nodes are mobile. However, in a dynamic system where the propagation delay may vary significantly, static values are not effective since large errors in timing may result.
Thus, what is needed is a method and apparatus for approximating a transmit time advance offset so that transmissions are not lost, and are received within the proper window.